Biomedical Engineering Reference
In-Depth Information
be produced, but they can only stay within circulation for 30 to 40 days, inducing a form
of anemia caused by a production of “inefficient” red blood cells. Causes of low tissue
oxygenation are anemia, destruction of hemoglobin, and reduced blood flow, among
others, all of which lead to increases in erythropoietin production. When there is a high
tissue oxygenation level, the production of erythropoietin is slowed, so that there is not an
excess of red blood cells/hemoglobin circulating within the cardiovascular system.
White blood cells (or leukocytes) are the primary cells that protect the body from for-
eign particles. There are two ways that leukocytes can perform this task: 1) leukocytes can
directly destroy foreign particles or 2) leukocytes can produce antibodies that aid in the
immune response. There are six types of white blood cells in the body, each with its own
specialized function. They are neutrophils, lymphocytes, monocytes, eosinophils, baso-
phils, and plasma cells. There are approximately 7000 to 8000 white blood cells/
Lof
blood. Of those cells, neutrophils account for more than 60% of them, lymphocytes account
for approximately 30%, monocytes account for approximately 5%, eosinophils account for
approximately 2.5%, basophils account for 0.5%, and plasma cells approximately 0.1%. All
white blood cells are formed in the bone marrow, except for lymphocytes and plasma
cells, which are formed in lymph glands/spleen (see Chapter 8). Compared to red blood
cells, white blood cells have a relatively short life span, which is typically only 5 days. The
exceptions to this are monocytes and lymphocytes; monocytes can migrate into tissues
within 1 day of being in the blood, and lymphocytes continually migrate between the
blood and the tissue for approximately 1 month. Monocytes that enter the tissue swell and
become macrophages and can stay in the extracellular space for months.
We will now briefly describe the immune responses of these cells. Neutrophils and
monocytes engulf invading particles directly through phagocytosis. These cells contain
many lysosomes to digest and destroy the foreign particles. Lymphocytes play a role in
acquired immunity, which is the inflammatory reaction against specific invading particles
within the body. Lymphocytes have a memory for foreign particles that have previously
entered into the body. Eosinophils exhibit a phagocytotic response toward parasites and
release hydrolytic enzymes to neutralize the parasite invasion. Basophils play an impor-
tant role in allergic reactions, and as such they can release histamine, serotonin, and hepa-
rin directly into the bloodstream. Plasma cells are antibody-producing cells and act in
response to specific antigens. Each plasma cell can form one antibody and acts as a mem-
ory for the immune system. Loss or low counts of white blood cells do not typically affect
the inflammatory response because the function of many of these cell types overlap.
Therefore, if one cell type is not functioning properly, a second type of white blood cell
can take up the slack. Leukemia is a cancer characterized by the overproduction of white
blood cells within the body. Unfortunately, these cells do not function properly and there-
fore multiple inflammatory processes are lost. The extent of leukemia dictates the amount
of inflammatory reactions that are lost.
Platelets (or thrombocytes) are the primary cells for hemostasis. They are cellular frag-
ments of megakaryocytes, which are derived from hematopoietic stem cells. Platelets are
typically ellipsoid in shape with a long axis of approximately 4
μ
μ
m and a short axis of
approximately 1.5
μ
m. The normal blood platelet concentration is approximately 250,000
platelets/
L. Platelets do not contain nuclei or many of the other common cellular orga-
nelles, but
μ
they do contain a large amount of calcium, ADP, prostaglandins, and
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